The present invention relates to an optical recording and retrieving disc capable of recording and retrieving information optically and usable as a most useful information file of large capacity. More particularly, the invention is concerned with an optical disc which permits recording, erasing and retrieving of information, and is usable as a superior medium in place of conventionally used magnetic discs.
Because no direct contact between the pickup and the information carrier is required, the optical information recording and retrieving system offers various advantages in protection of the pickup and information carrier from wear and damage, high speed of access to the information and so forth.
In the case where the information carrier carries the information at a high density, however, it is necessary to control the irradiating light spot to accurately follow up the information track.
To cope with this demand, some methods have been proposed one of which is a method generally called 3 (three) beam method. According to this three-beam method, the irradiating light beam is decomposed by a diffraction grating into three spots: namely, 0 (zero) degree light spot, +1 degree light spot and -1 degree light spot. The irradiation is made onto the information track in such a manner that these three spots are arrayed in a line which is inclined by a small angle from the information tracks so that the 0 degree light spot irradiates the mid point on the information track while the +1 degree light spot and the -1 degree light spot irradiate points which are slighly deviated to the left and right, respectively, from the information track.
The traction detecting section is composed of a portion which retrieves the signal upon detecting the reflection of the 0 degree light spot, and includes also a first sensor for sensing the reflection of the +1 degree light spot and a second sensor for sensing the reflection of the -1 degree light spot. It is possible to detect the direction and amount of deviation of the irradiating light beam from the information track through detecting the difference between the outputs from the first and second sensors and, hence, to effect accurate tracking control by controlling the irradiating light beam in such a manner as to nullify the difference between the outputs of two sensors. This method, however, suffers problems such as necessity for the means for decomposing the irradiating light beam into three spots and reduced intensity of irradiation of track due to the decomposition of the light beam, although it affords a stable tracking.
Another known method for the tracking is a method called "far field method". This method applies to an information carrier in which informations are recorded on a substrate having a refractive index n in the form of concavity and convexity or step having a height difference. In operation, a light spot of a wavelength .lambda. is applied to the top surface and bottom surface of a portion of the track having a height difference .DELTA.l, so that a phase difference .psi. expressed by the following formula is formed between the light reflected by the top surface and the light reflected by the bottom surface. EQU .psi.=4.pi..multidot..DELTA.l.multidot.n/.lambda.
These two reflected light beams make interference and diffraction to produce a far field pattern the intensity distribution of which is varied in accordance with a change in the relative position between the light spot and the step. The change of the intensity distribution is detected by a first optical sensor and a second optical sensor which are disposed in the tracking detection section. When the light spot is irradiating the stepped portion of the track in a symetrical form, the far field pattern of the reflected light exhibits a symetrical intensity distribution. The first and second optical sensors are arranged such that they produce outputs of an equal level when the far field pattern a symmetrical intensity distribution. Therefore, if the light spot deviates from the point of symmetry of the stepped portion, the intensity distribution in the far field pattern changes to cause a change in the difference between the outputs from the first and second sensors. By detecting this change, it is possible to measure the direction and amount of deviation of the light spot from the track and, hence, it is possible to accurately hold the light spot accurately in the information recording track groove, i.e. to effect an accurate tracking, by controlling the position of the light spot so as to nullify the difference between the outputs from the two sensors. This tracking method is superior in that the tracking can be made effected accurately with a single light spot.
Electronik 1978, Vol. 27, No. 15, pages 31-34 discloses an optical recording and retrieving disc having a substrate with pre-grooved spiral tracks. Pits are formed in a recording film on the substrate in order to record information. The phase difference between the light reflected from the non-recording portion of the film and from the pits is utilized for tracking.